When all other sources of error and distortion have been removed or minimized by correction or compensation, standard NTSC color video picture images become limited in quality by perceptibility of the line scan structure. Subjective visibility of the line scan structure is a direct consequence of the limited number of horizontal scan lines in the standard composite picture, and is further a direct consequence of field by field interlace. A conventional television frame at a 30 Hz repetition rate is composed of two fields, e.g. F0 and F1. Each field includes 262.5 scan lines, each of which are separated by an unilluminated strip or band. Successive fields are offset so that the scan lines of the next field occupy the unilluminated strips of the present field. This arrangement is followed to minimize perception of 30 Hz flicker in the resultant picture display.
However, with development of high resolution display devices and large screen formats, the visibility of the basic line scan structure has become the most pronounced limitation of the conventional scan format and has led recently to a proliferation of proposals for high definition television or "HDTV". A main drawback of most HDTV formats is that they are not downwardly compatible with existing transmission and reception equipment, equipment which represents an enormous investment.
One approach for reducing the visibility of the line scan structure of the television image calls for estimating or interpolating picture elements for additional scan lines from the picture elements already present in the picture image scanned in the conventional format. This prior approach is known in the art as "time compression" or "scan line doubling", and it calls for doubling the number of scan lines from 262.5 lines per field to 525 lines per field. Thus, 525 scan lines are presented each 60th of a second.
One prior approach to pixel interpolation is carried out by an intra-field or spatial domain process. The pixel for the unilluminated band between two scan lines is derived as the average of the pixel amplitude (and hue) of the pixel in the scan line directly above and of the pixel in the scan line directly below. This approach may be followed with relatively simple circuit implementation. Its main drawback is the reduced resolution or softness of the resultant picture image in the vertical dimension at edges and some perceptible 30 Hz vertical domain flicker in the instance of sharp vertical transitions within the picture image.
Another prior approach to line doubling operates on inter-field basis. In this approach, the pixel to be interpolated for a new scan line in the present field is obtained as the pixel value from the scan line of the prior field at the same spatial location of the picture image. This inter-field approach requires a field period delay and is much more expensive to implement than the intra-field approach. However, when there is no motion in the picture, a very high resolution picture display results. When there is motion, the inter-field approach completely breaks down. In the presence of motion, the objects in motion are accompanied by double image ghosts or aliases, ragged sawtooth patterns on vertical edges in the picture and flickering at the 30 Hz rate along motion transition edges. With an otherwise noise-free picture, a motion value of 2 IRE units, for example, will result in visible double image aliases with this prior approach.
Thus, the inter-field approach to pixel interpolation has not been successfully used in line doublers without some mechanism for motion adaptation. Motion adaptation circuits add further expense to this approach. With a motion adaptation circuit, it has then been possible to construct a line doubler which switches from the higher resolution inter-field approach to the lower resolution intra-field approach whenever motion is detected within the picture image. Such an approach is described and discussed in conjunction with FIG. 13 of Powers U.S. Pat. No. 4,400,719 entitled "Television Display System with Reduced Line-Scan Artifacts".
Since most television picture content derived from reality includes a significant motion content, the prior art inter-field line doublers handled the motion content by switching to intra-field spatial domain interpolation, for example. A significant drawback of the prior art inter-field line doublers is that they have needed to be switched at a very low motion level to the spatial domain interpolation in order to avoid objectionable artifacts in the resultant picture. With such a low a switch threshold being required, the switching has been triggered by noise in the picture and has resulted in a noisier picture with intra-field interpolation.
A temporal median filter has been proposed for use within a television receiver in a paper by C. Hentschel entitled "Linear and Non-Linear Procedures for Flicker Reduction" presented at the 1987 IEEE International Conference on Consumer Electronics and published in the Digest of Technical Papers at pages 174 and 175. However, in that approach two new fields were derived by temporal median filter interpolation and inserted in a field-doubled display. There is no teaching or suggestion in the published paper of a temporal median filter for use within an inter-field scan line doubler structure.
Thus, a hitherto unsolved need has arisen for an inter-field line doubler which enables temporal interpolation of pixels at a higher motion threshold than heretofore without generation of objectionable artifacts.
Objects in motion within the video picture must still be detected, so that temporal pixel (field difference) interpolation processes are discontinued in favor of spatial pixel (line difference) interpolation processes. While field-recursive motion detection has been proposed by the Tonge et al. U.S. Pat. No. 4,730,217 in order to bridge spectral gaps occurring in certain conditions of motion within the picture, the recursive approach taken therein provides for recirculation or feedback of the overall motion control signal itself, rather than one component or aspect thereof. The Tonge et al. overall recirculation technique employed field delay recirculation. The Tonge et al. definition of "field" is unclear. If "field" as used by Tonge et al. meant 262 lines, the resultant recirculation would cause a spread or crawl upwardly into the vertical domain. If "field" as used by Tonge et al. meant 263 lines, downward spread or crawl into the vertical domain resulted. In any event, the Tonge et al. approach resulted in an undue vertical spreading of the motion control signal and did not aid or foster development of a motion control signal which is related in amplitude or value to overall magnitude of the motion event being detected. The practical consequence of the Tonge et al. approach was that the pixel interpolator switched to a spatial domain interpolation process too frequently and for too long a time, thereby resulting in a picture of lesser resolution than necessary.
Thus, a further hitherto unsolved need has arisen for an improved motion detector which enables development of a proportional interpolation process switching control signal while still covering spectral gaps occurring from some picture motion conditions.